1. Technical Field
The present invention relates in general to information-retrieval methods and systems. In particular, the present invention relates to magnetic media. More particularly, the present invention relates to magnetic tape drive systems. Still more particularly, the present invention relates to methods and systems for detecting the end of magnetic storage tapes within magnetic tape drive systems.
The present invention is additionally related to motion and positional control of magnetic tape in a reel-to-reel tape drive. Such control of magnetic tape motion and position in reel-to-reel tape drives is described in detail in U.S. Pat. No. 4,015,799 and U.S. Pat. No. 4,125,881, both assigned to the assignee of this application and incorporated herein by reference in their entireties.
2. Description of the Related Art
Magnetic tape drives are commonly utilized for controlling the movement of magnetic tape past a read or write head, and/or to allow automatic rewinding. Magnetic tape, usually referred to simply as "tape," is typically composed of a thin strip of polyester film coated tape with a magnetizable layer on which data can be stored. Because the tape includes a continuous length of data storage material and because the read/write head on a magnetic tape drive cannot simply "jump" to a desired point on the tape without the tape first being advanced to that point, the tape must be read or written sequentially, not randomly, as is typically accomplished with floppy or hard disks.
Most tape drives to date have been designed for relatively thick media (i.e., thick tape). In a typical tape drive, the end of a tape being driven through the tape drive is detected by literally running off the end of the tape. The tape itself is physically utilized to pull a pantocam pin out of the reel upon which the tape is wound and a sensor is then activated to indicated that the end of the tape has been reached. Such techniques function properly with thick "beefy" tape, but is inefficient when utilized with thinner tape. Such thinner tape is increasingly being utilized to store and record data. Utilizing these old techniques for detecting the end of the tape causes undue wear on the pantocam linkage, contributing to high pantocam field replacement rates. In addition, too much stress is placed on thinner tape when utilizing such end-of-tape detection techniques.
In the process of reading or writing data on a magnetic tape, an uncertainty in position results from tape stretching and the entrainment of air in the wraps of tape on the reels. A motion from end to end can result in an uncertainty of several meters of tape. Consequently, the tape position measured by the counters connected to tachometers on the reel motors can not be used to reliably predict the end of tape. In prior art tape drives, a simple mechanism solved this problem. A flag on the tape threading mechanism (i.e. the pantocam) was utilized to break an optical sensor in order to signal a control processor that the pantocam was in the position where the leader block attached to the tape was fully inserted in the take up reel. To find the end of the tape, the pantocam motor was run for a short period of time in order to create some slop in the pantocam gearbox through backlash in the gears.
The end of the tape was found by rewinding tape from a take-up reel back to the supply reel in the cartridge under velocity control. The indication that the end of the tape was reached was provided when the optical sensor on the pantocam became active as the flag on the pantocam arm was jerked out of the sensor as the end of the tape was reached and the pantocam abruptly yanked away from the take-up reel. Such a system operated successfully as long as the mechanical strength of the tape was reliable to yank the pantocam arm and the lead block out of the take-up reel spool.
Based on the foregoing, it can be appreciated that a need exists for a method and system which overcomes the problems associated with prior art tape drive systems, namely the inability to properly detect the end of tape within a tape drive system, without damaging the tape itself, particularly thin tape which does not possess the mechanical strength to repeatedly pull out the pantocam for thousands of load/unload cycles. A need also exists for a method and system which would detect the end of tape within a tape drive system utilizing a single reel cartridge, while reducing undue wear on the pantocam caused by an abrupt jerk when the end of tape is finally detected. A need also exists for a method and system which verifies that the end of tape has been reached and the tape can be successfully unthreaded without damaging the tape.